After two hours were lost at The Celtic Manor Resort due to thunderstorms in the afternoon, it was close to darkness when Horsfield sank the putt that made him the 2020 Race to Dubai's first multiple winner.
A bogey free 67 in Wales took the Englishman to 18 under and handed him a second win on the UK Swing after his triumph at the Hero Open two weeks ago.
Horsfield started the day one shot behind Syme but he had a share of the lead after the first and never looked back as he became the first ever player on the European Tour to win an event, miss the cut, and then win again in three consecutive weeks.
The victory takes 23-year-old to the top of the UK Swing mini Order of Merit, into the top ten on the Race to Dubai Rankings presented by Rolex and into the top 100 on the Official World Golf Ranking - his highest ever placing in the global standings.
"I never really felt nervous or felt uncomfortable, maybe because I was in this situation a few weeks ago," he said.
"I’m a little more relaxed than last time I did it. It was good. I felt like I played really solid all day and gave myself a lot of opportunities.
“After I missed the cut last week I was never concerned. I felt like after your first win and everything that was going on I felt a little mentally drained and I probably should have decided to take the week off. I just wasn’t all there but I knew my game was in good shape so I basically just forgot about last week."
He added: "It was pretty dark coming down 18. That putt on 18 I couldn’t really see much of what was going on but I was set in stone on finishing that round. I didn’t want to get up at whatever time and finish on Monday morning.”
Detry holed a 30 footer on the second to close the gap at the top and he was soon within one as Syme failed to get up and down on the first and gave up just a second bogey of the week.
Horsfield hit the front with a long putt at the third and another birdie at the fourth meant he was two ahead.
Detry holed another monster on the seventh to get in a share of second but Horsfield holed from 12 feet at the same hole to extend his advantage.
After the two hour delay, Syme's first challenge was a short par putt on the eighth which he made, and he got up and down on the ninth to trim the lead back to two.
Detry took advantage of the par five 11th to get to 15 under and when Syme made a birdie on the same hole from four feet, the pressure was building on Horsfield.
It ramped up further still as Detry holed a very slippery 15 footer on the 14th but Horsfield holed from 16 feet on that hole to give himself a bit of breathing room.
Syme took on the short par four 15th but needed a further four blows from in front of the green and, just like two weeks ago in Birmingham, Detry was Horsfield's closest challenger.
Horsfield needed a smart par save on the 17th to stay at 18 under and with his two challengers parring their way home, he was the champion.
Pieters finished his round of 67 with four consecutive birdies in his first start since the coronavirus hiatus, while a closing 68 made it back to back top 20 finishes for Johnston.
Catlin chipped in at the last in a 65, Horsey and Välimäki signed for rounds of 66 and Meronk finished with a 69, while Swede Sebastian Soderberg rounded out the top ten at 13 under after a 71.